This invention relates to a centrifugal impact crusher, and in particular to such a crusher in which 100% of the material fed into the crusher is impacted against the crushing anvils and in which the amount of dust is significantly reduced.
Centrifugal impact crushers have successfully been used for crushing a wide range of materials. A use that has recently been suggested for centrifugal impact crushers is in the production of cement. Cement has traditionally been ground from clinker in ball mills using a two-step process. The clinker is first rough ground in a first section of the ball mill having large balls, and the ground material from the first section is then reground in a second section having smaller balls. When ball mills are used to grind large particles, such as occur with clinker, they consume excessive amounts of energy, and thus they are not well-suited for this purpose. German patent DE3815217 suggests using a centrifugal impact crusher for the first section in crushing clinker, since centrifugal impact crushers handle material of this size much more efficiently than ball mills. However, the smaller second stage ball mill required to grind fine enough to produce cement does not handle large material well and the first stage device must reduce all of the material passing through it in the required amount. Prior art centrifugal impact crushers were designed to crush materials such as rock where a wide size range of crushed material is not only allowable, but actually desired. A substantial amount of material deposited onto the rotating table of prior art centrifugal impact crushers does not even strike the crushing anvils and thus is uncrushed. This means that when centrifugal impact crushers are used for the first stage of clinker reduction, the material must be sorted and a portion of it reground which increases the cost of using centrifugal impact crushers for clinker. Furthermore, cement creates tremendous amounts of dust when it is crushed in the violent manner that exists in a centrifugal impact crusher, and this dust creates significant problems.
While complete reduction and dust handling are critical when crushing clinker, dust creates problems when other materials are crushed as well. In many materials dust is handled by wetting the material. While wetting is obviously unworkable with cement, water makes sorting of other materials more difficult and it increases wear. In addition, when rock is crushed to sand, it is important that dust be separated from the sand and this cannot be done when the material is wetted during crushing.
The foregoing shortcomings and limitations of prior art centrifugal impact crushers are overcome in the subject invention by providing a table having spaced-apart upper and lower plates that have impeller blades sandwiched between them. The upper plate has a central opening through which material is deposited onto the table. Thus, the table is enclosed which forces substantially all of the material thrown off of it by centrifugal force to be impacted against the anvils. In addition, an annular retainer plate fits above the anvils and extends radially inward to the periphery of the upper plate. The retainer plate covers what little area there is for material that is thrown off of the enclosed table to miss the anvils.
The impellers have tabs protruding from their upper and lower surfaces which fit in conforming receptacles located in the upper and lower plates. The upper and lower plates are joined to one another by bolts or pins, and when the bolts are tightened or wedges inserted into the pins the tabs are seated in the receptacles and the plates and impellers form a rigid table assembly. The tabs cover a substantial portion of the impellers and, as a result, impeller attachment is spread over a large surface area which prevents the impeller connector breakage that is common with devices of this type due to the centrifugal force created by the high rotational speed and due to material striking the impellers.
In order to reduce dust and ensure uniform fine crushing, recirculation plenums extend from an outlet plenum at the bottom of the crusher shell to an infeed plenum through which material is fed into the crusher. The rotating table in the crusher acts as a fan which creates a negative pressure above the table and a positive pressure below the table. This pressure differential between the infeed plenum above the table and the outlet plenum below the table draws air into the recirculation plenums. Since the recirculation plenums receive air from beneath the table, dust and particulate matter resulting from the crushing is entrained in the air and this material passes through the recirculation plenums and back into the crusher. The recirculation plenums have outlets in them which permit particle-laden air to be removed from the plenums and the particles removed by filters or separators if desired. A flow restrictor located above the infeed plenum restricts the air that enters the crusher with the material in order to establish significant flow through the recirculation plenums. The infeed plenum is connected to the central opening in the upper plate of the table so that all of the material and dust-laden air in the infeed plenum reaches the table.
Accordingly, it is a principal object of the subject invention to provide a centrifugal impact crusher in which 100% of the material deposited in the device is thrown off of the table and impacted against the crushing anvils.
It is a further object of the subject invention to provide such a device in which the table is enclosed.
It is a still further object of the subject invention to provide such a device having a retaining plate over the anvils which prevents material from passing over them.
It is a still further object of the subject invention to provide a centrifugal impact crusher having an air recirculation system that recirculates air and entrained particles from the bottom of the crusher back into the infeed plenum.
It is a yet further object of the subject invention to provide such a crusher in which the air and entrained particles in the recirculation system can be filtered to remove the particles.
The foregoing and other objectives, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings.